Digital displays based on liquid crystal technology exhibit many advantageous properties. For example, they are thin and lightweight, exhibit relatively fast switching speed, operate on low voltages, consume a minimum amount of power and provide good viewing contrast. Liquid crystal materials do not have inherent memory, however. That is, an energized liquid crystal display cell will not remain permanently energized once the energizing voltage has been removed.
The lack of inherent memory in a display material is a problem if that material is to be used, for example, in computer terminal displays. This is because it is desirable to incorporate into such displays a matrix addressing scheme wherein a given display cell is addressed by applying coincident signals to a pair of row and column conductors associated with that cell, and when such an addressing scheme is employed, it is not possible to apply a constant voltage to each energized cell. Advantageously, however, in order for a liquid crystal display cell to remain energized, the signal applied to it does not have to be constant. Rather, a liquid crystal cell will remain energized as long as the rms, i.e., average, voltage thereacross is above some minimum level. Thus, the energized cells in a matrix-addressed liquid-crystal display can be maintained energized by periodically applying a refresh pulse to each energized site in successive rows (or, alternatively, in successive columns) on a multiplexed basis.
As long as the number of rows (or columns) to be multiplexed is not too large, the multiplexing technique just described works quite well. As the number of multiplexed rows (or columns) increases, however, the multiplexing frequency must be increased. Ultimately, this will result in a decrease in the rms voltage across each energized cell and beyond a certain point, the viewing quality begins to deteriorate.
One prior art alternative is to use thermally-addressed pleochroic dye switching to provide a liquid crystal display which has built-in memory. See, for example, S. Lu et al, "Thermally-addressed Pleochroic Dye Switching LCDs," SID Symposium Digest, pp. 238-239 (1982); and S. LeBerre et al., "A Flat Smectic Liquid Crystal Display," SID Symposium Digest, pp. 252-253 (1982). This approach, has several drawbacks, however, including a high power requirement and relatively slow writing speed.
Another prior art solution is to couple each liquid crystal display device with a non-linear device and capacitive storage element. See, for example, D. E. Castleberry et al., "A 5 inch.times.7 inch Varistor Controlled LC Matrix Display, " SID Symposium Digest, pp. 246-247, (1982); R. W. Streater et al., "MIM Addressed LCDs: Status and Prospects," SID Symposium Digest, pp. 248-249 (1982); M. Matsuura et al., "A liquid Crystal Display Device with Thin Film Transistors," SID Symposium Digest, pp. 34-35 (1982); F. C. Luo et al., "Hybrid-Processed TFT Matrix Circuits for Flat Display Panels," SID Symposium Digest, pp. 46-47 (1982). This approach still entails multiplexing, but the addition of the capacitive storage element allows the multiplexing frequency to be increased without critically reducing the rms voltage across the liquid crystal display itself. Unfortunately, the processing yields for this technique are reported to be low, and estimates are that displays embodying it will not be commercially available for quite some time in the future.
Yet another prior art solution is to provide separate sets of row and column conductors for respective sections of the display, thereby allowing the different sections to be addressed independent of one another. An example of this is the Model LM-24002G Graphic Display Unit manufactured by Sharp Electronics Corporation. Prior art displays following this approach, however, have been able to include only two such independently addressable sections--each having its column conductors extend to the center of the display from a respective one of two parallel edges thereof. This is because the prior art does not provide a practical technique for providing electrical connection from an edge of the display to conductors which are associated with an interior section or sections thereof.